Tony Blair has written ''I’ve always been more interested in religion than politics’’, a striking thought for a prime minister, so I ask him why. Religion, he says, engages with ''the fundamental truths about life’’. He feels he is now ''deeply familiar with the rules of politics’’; in religion, ''there is so much that is still unexplored’’.

Yes, but many would say that who are not themselves religious. He is. Why? He is interestingly discreet. He does not want to talk about God, or about the Church (if you look up ''Church’’ in the index of his autobiography, it says only ''Church, Charlotte p.333’’). He speaks of Jesus as a man who was ''prepared to challenge conventional wisdom when he thought it was wrong’’. More spiritual thoughts he keeps to himself. What interests him is that, as ''a person of faith’’, he has ''a connection with people of faith’’. He sees himself as their interpreter.

Mr Blair cites a meeting at the Davos Economic Forum a few years ago. There were representatives of four different faiths on the platform, each with what he calls ''an exclusive truth claim’’ for their religion. He asked them if they thought that only their faith led to salvation. ''It was interesting to see them reacting as politicians react. I spotted all the techniques of walking round it.’’

He can’t answer his own question fully, he admits. As a Catholic convert, he ''accepts the doctrine of the Catholic Church’’, but ''I’m not a doctrinal ideologue’’. He feels ''no great revulsion, quite the opposite’’ for the Church of England, which he left. He became a Catholic because of his Catholic wife, Cherie, and their family: ''I didn’t really analyse a great deal. I just felt more at home there.’’

His approach to religion relates to social change. ''When I was growing up in the North of England, there was really only one faith,’’ he says. His best friend at Durham Choristers School was, in fact, Jewish, but ''I literally never even thought of that’’. The mono-culture was Christian. Recently, his son Leo celebrated his 12th birthday ''and the children round the table had three or four different faiths’’.

Under the benign influence at Oxford of the Anglican priest Peter Thompson, young Tony came to believe that faith and reason could be reconciled. From this he concluded that different faiths, especially the ''Abrahamic’’ religions of Judaism, Christianity and Islam, could build on what they have in common. Now he reads the scriptures of other faiths, and finds his own enriched. In particular, he reads the Koran.

''I see the Koran very much as an outsider. It stands in the great prophetic tradition of trying to return people to the basic principles of spirituality. Taken for its time, it was an extraordinarily progressive declaration of principle. It is also extraordinary for a Christian to read: for example, there are more references to Mary than in the Gospels. The tragedy is that it has been so warped and misapplied.’’

And here, Tony Blair has grown sterner. After September 11, 2001, he now thinks, he underestimated the power of the bad ''narrative’’ of Islamist extremists. That narrative – that ''The West oppresses Islam” – ''is still there. If anything, it has grown.’’ It seeks ''supremacy not coexistence’’. He fears that ''The West is asleep on this issue’’, and yet it is the biggest challenge. In Africa, all the good things he sees through his Africa Governance Initiative face ''this threat above all others’’. In ''Sudan, Mali, Nigeria, outbursts in Tanzania and Kenya’’, sectarian Islamist extremism is the great and growing problem. By implication, Mr Blair seems to doubt President Obama’s outreach to Islam, because it tends to deal with the wrong people. Since Obama’s Cairo speech in 2009, ''the whole context has changed’’. The Muslim Brotherhood is taking over large parts of the Arab world, and ''the people without the loudest voices are desperate for our leadership’’.

''We must engage, but also challenge,’’ he warns. The Middle East ''won’t achieve democracy unless it understands that democracy is a way of thinking as well as voting. The key question is how the majority treats the minority.’’ The West, he says, has been too slow to help the people of Iran: ''It is a great civilisation. The people would undoubtedly boot their government out at the ballot box if they could. It is important they know we are prepared to help them. A Persian spring would be very welcome.’’

But have you considered, I ask, that you might be wrong about Islam? What if it is not, at root, a religion of peace? He has thought about this but doesn’t accept it. He makes a comparison with Christianity. ''At Mass, at the end of the Bible readings, we say 'This is the word of the Lord’. We now take it as the spirit of Biblical teaching. We don’t take every element of it as literal. That process took us a long time.’’ Islam is wrestling with the same process today.

Let’s bring the subject home: how does this apply to Muslims here? Mr Blair regrets that the ''Prevent’’ strategy which he devised became unfashionable. ''We mustn’t accept radicalism by accepting its narrative and disputing only its [violent] methods.’’

But he also believes that the anti-religion, Richard Dawkins crowd make everything worse. The extreme atheists ''require religious fundamentalists’’ to make their argument for them, so ''We must push back against aggressive secularism’’.

Very well, then, I say, look at gay marriage, a proposal that troubles many adherents of all the main religions. No comfort for the faithful here: Mr Blair is out of line with his adopted Church. ''I understand why people take a different view,’’ he says, but he is in favour of gay marriage. Indeed, it is not really possible to find a public policy issue where he takes a specifically religious view against the prevailing secularism. It is, rather, a broader point: he thinks religion is a benign force in a modern liberal order, not a hostile one.